This core facility will generate mutant mice for UCSF Cancer Center investigators. Transgenic mice will be created by microinjecting DNA constructs directly into the pronuclei of fertilized mouse oocytes. Chimeric mice will also be created by microinjecting mouse embryonic stem (ES) cells into 3.5-day old blastocysts. Through these two mechanisms, the Core will facilitate the generation of transgenic, knockout, knock-in and other types of mutant mice for cancer-related research at UCSF. The Core will also perform embryo transfers to allow rederivation of imported and/or infected strains of mice into the specific pathogen-free mouse facility. DNA constructs for microinjection into fertilized oocytes will be purified by the core prior to microinjection using standardized and proven methodology. ES cells will be provided as a freshly-prepared suspension immediately before microinjection. Chimeric and transgenic mice will be born in the core facility colony and then transferred within the UCSF barrier to the relevant investigators? colonies.